Yes, I'm aware that spell books are rare loot and if you want them all, it is more useful to buy them. But I don't want to - it is a roleplaying thing. I'm not a mage anyway and don't have the maciga for most of the spells, I want them all because I'm a hopeless idealist. And I don't mind that it will take time, Skyrim is still one of my favorite games and exploring and clearing new dungeons is what's fun, so having the possibilyty of finding a new spell adds to that. Therefore I do not want to buy even one spell that can be obtained otherwise. Which are the spells that never drop, even inside Black Books, and can only be obtained by buying them?
According to the uesp wiki (http://www.uesp.net/wiki/Skyrim:Spells),
Nearly all spell tomes can also be found randomly as loot. The only spell tomes that can not be found randomly are quest-specific spell tomes (those that can not be purchased), tomes added by Dawnguard or Dragonborn, and master-level tomes. The appearance of random tomes is dependent upon your character's overall level (but is not related to your skill in the magic school, unlike tome purchases)